# Django 2.0.dev20170216164632 documentation

# Lookup API reference

This document has the API references of lookups, the Django API for building the `WHERE` clause of a database query. To learn how to use lookups, see Making queries; to learn how to create new lookups, see Custom Lookups.

The lookup API has two components: a `RegisterLookupMixin` class that registers lookups, and the Query Expression API, a set of methods that a class has to implement to be registrable as a lookup.

Django has two base classes that follow the query expression API and from where all Django builtin lookups are derived:

- `Lookup`: to lookup a field (e.g. the `exact` of `field_name__exact`)
- `Transform`: to transform a field

A lookup expression consists of three parts:

- Fields part (e.g. `Book.objects.filter(author__best_friends__first_name...`);
- Transforms part (may be omitted) (e.g. `__lower__first3chars__reversed`);
- A lookup (e.g. `__icontains`) that, if omitted, defaults to `__exact`.

## Registration API

Django uses `RegisterLookupMixin` to give a class the interface to register lookups on itself. The two prominent examples are `Field`, the base class of all model fields, and `Aggregate`, the base class of all Django aggregates.

class `lookups.``RegisterLookupMixin`

  A mixin that implements the lookup API on a class.

  classmethod `register_lookup`(lookup, lookup_name=None)

    Registers a new lookup in the class. For example `DateField.register_lookup(YearExact)` will register `YearExact` lookup on `DateField`. It overrides a lookup that already exists with the same name. `lookup_name` will be used for this lookup if provided, otherwise `lookup.lookup_name` will be used.

  `get_lookup`(lookup_name)

    Returns the `Lookup` named `lookup_name` registered in the class. The default implementation looks recursively on all parent classes and checks if any has a registered lookup named `lookup_name`, returning the first match.

  `get_transform`(transform_name)

    Returns a `Transform` named `transform_name`. The default implementation looks recursively on all parent classes to check if any has the registered transform named `transform_name`, returning the first match.

For a class to be a lookup, it must follow the Query Expression API. `Lookup` and `Transform` naturally follow this API.

## The Query Expression API

The query expression API is a common set of methods that classes define to be usable in query expressions to translate themselves into SQL expressions. Direct field references, aggregates, and `Transform` are examples that follow this API. A class is said to follow the query expression API when it implements the following methods:

`as_sql`(self, compiler, connection)

  Responsible for producing the query string and parameters for the expression. The `compiler` is an `SQLCompiler` object, which has a `compile()` method that can be used to compile other expressions. The `connection` is the connection used to execute the query.

  Calling `expression.as_sql()` is usually incorrect - instead `compiler.compile(expression)` should be used. The `compiler.compile()` method will take care of calling vendor-specific methods of the expression.

  Custom keyword arguments may be defined on this method if it’s likely that `as_vendorname()` methods or subclasses will need to supply data to override the generation of the SQL string. See `Func.as_sql()` for example usage.

`as_vendorname`(self, compiler, connection)

  Works like `as_sql()` method. When an expression is compiled by `compiler.compile()`, Django will first try to call `as_vendorname()`, where `vendorname` is the vendor name of the backend used for executing the query. The `vendorname` is one of `postgresql`, `oracle`, `sqlite`, or `mysql` for Django’s built-in backends.

`get_lookup`(lookup_name)

  Must return the lookup named `lookup_name`. For instance, by returning `self.output_field.get_lookup(lookup_name)`.

`get_transform`(transform_name)

  Must return the lookup named `transform_name`. For instance, by returning `self.output_field.get_transform(transform_name)`.

`output_field`

  Defines the type of class returned by the `get_lookup()` method. It must be a `Field` instance.

## `Transform` reference

class `Transform`

  A `Transform` is a generic class to implement field transformations. A prominent example is `__year` that transforms a `DateField` into a `IntegerField`.

  The notation to use a `Transform` in an lookup expression is `<expression>__<transformation>` (e.g. `date__year`).

  This class follows the Query Expression API, which implies that you can use `<expression>__<transform1>__<transform2>`. It’s a specialized Func() expression that only accepts one argument. It can also be used on the right hand side of a filter or directly as an annotation.

  `bilateral`

    A boolean indicating whether this transformation should apply to both `lhs` and `rhs`. Bilateral transformations will be applied to `rhs` in the same order as they appear in the lookup expression. By default it is set to `False`. For example usage, see Custom Lookups.

  `lhs`

    The left-hand side - what is being transformed. It must follow the Query Expression API.

  `lookup_name`

    The name of the lookup, used for identifying it on parsing query expressions. It cannot contain the string `"__"`.

  `output_field`

    Defines the class this transformation outputs. It must be a `Field` instance. By default is the same as its `lhs.output_field`.

## `Lookup` reference

class `Lookup`

  A `Lookup` is a generic class to implement lookups. A lookup is a query expression with a left-hand side, `lhs`; a right-hand side, `rhs`; and a `lookup_name` that is used to produce a boolean comparison between `lhs` and `rhs` such as `lhs in rhs` or `lhs > rhs`.

  The notation to use a lookup in an expression is `<lhs>__<lookup_name>=<rhs>`.

  This class doesn’t follow the Query Expression API since it has `=<rhs>` on its construction: lookups are always the end of a lookup expression.

  `lhs`

    The left-hand side - what is being looked up. The object must follow the Query Expression API.

  `rhs`

    The right-hand side - what `lhs` is being compared against. It can be a plain value, or something that compiles into SQL, typically an `F()` object or a `QuerySet`.

  `lookup_name`

    The name of this lookup, used to identify it on parsing query expressions. It cannot contain the string `"__"`.

  `process_lhs`(compiler, connection, lhs=None)

    Returns a tuple `(lhs_string, lhs_params)`, as returned by `compiler.compile(lhs)`. This method can be overridden to tune how the `lhs` is processed.

    `compiler` is an `SQLCompiler` object, to be used like `compiler.compile(lhs)` for compiling `lhs`. The `connection` can be used for compiling vendor specific SQL. If `lhs` is not `None`, use it as the processed `lhs` instead of `self.lhs`.

  `process_rhs`(compiler, connection)

    Behaves the same way as `process_lhs()`, for the right-hand side.